The assignee of the present invention manufactures and deploys spacecraft for observation of the earth and other celestial bodies, and to provide broadcast and communications services, for example. To perform their mission, the payloads and solar panels of such spacecraft must be oriented and maintained in a particular orientation with respect to the earth (or other celestial body) or with respect to the earth and sun. For example, the general requirement for three-axis, body-stabilized spacecraft operating in geostationary orbit is to orient the spacecraft such that the payload is directed toward the earth and the axis of rotation of the solar array is orthogonal to the earth's equatorial plane. In such orientation, the solar arrays are enabled to rotate in such a manner to compensate for the spacecraft's motion with respect to the sun. Maintenance of this desired three-axis attitude is provided by way of attitude and rate sensors coupled to torque generators through an attitude determination and control subsystem, which includes an onboard computer referred to as the spacecraft controller. Attitude sensors may comprise celestial body observers, such as earth sensors, sun sensors and star trackers. Rate sensors may comprise such devices as digital integrating rate assemblies or gyros. Torque generators may comprise such devices as thrusters, magnetic torquers or momentum or reaction wheels.
U.S. Pat. No. 7,874,519, assigned to the assignee of the present invention and U.S. Pat. No. 6,695,263 disclose techniques for achieving or reacquiring three-axis attitude control. To achieve or reacquire three-axis attitude control, generally requires determining a direction of the sun with respect to a spacecraft coordinate frame. Satellites are configured with digital or analog sun sensors for that purpose, and six to eight such sensors may typically be required for a geosynchronous satellite. Alternatively or in addition, when orbital parameters and time are known, the sun's vector with respect to the spacecraft orbital position can be calculated, and so a spacecraft with a star tracker can be re-oriented to be sun-pointing.
The presently disclosed techniques enable reducing the number of required sun sensors and also enhance satellite autonomy.